1. Field of the Invention
The present invention relates to a technique of cleaning a mold for rubbers. More specifically, it relates to a rubber composition for cleaning a mold whose surface is soiled by repeated vulcanization, and a method for cleaning the soiled mold.
2. Description of the Related Art
In the field of rubber molding, the soil of a mold for vulcanization have been known for a long time, and the soil of the mold increases with the repetition of the vulcanization, so that some problems are present. For example, the molded articles of rubber are seriously impaired in points of quality, appearance, dimensional accuracy and the like, and the release properties of the molded articles from the mold deteriorate and the life of the mold also shortens.
Under such circumstances, many investigations have been carried out to prevent the mold from soiling. For example, there have been suggested a method comprising the step of adding an organic phosphoric ester and a carboxylate of a cyclopentadiene derivative to a solution for the external surface of the mold in Japanese Patent Publication No. 44980/1984, a method which comprises the step of treating the surface of the mold with a compound having silicon and an isocyanate group in its molecule in Japanese Patent Application Laid-open No. 113518/1987, and techniques for inhibiting the soil of the mold by blending a rubber composition with a cyclic polybutadiene (Japanese Patent Application Laid-open No. 63144/1979), an organic phosphoric ester salt (Japanese Patent Publication No. 53812/1982), a mercaptobenzo-1,3-diazol salt (Japanese Patent Application Laid-open No. 107943/1979), a metal carboxylate of a cyclopentadiene derivative (Japanese Patent Application Laid-open No. 126251/1979), 2-mercaptobenzoimidazole or its zinc salt (Japanese Patent Publication No. 20585/1986) or a specific silica (hydrated silicic acid, Japanese Patent Application Laid-open No. 238143/1988).
These methods can decrease the soil of the mold, but they cannot completely prevent the soil. Therefore, the mold has periodically been cleaned by, for example, a shot blast method using glass beads and an acid or an alkali cleaning method, but these mold cleaning techniques tend to damage the mold, accelerate the soil of the mold after the cleaning, and corrode the mold. In addition, for the cleaning, the mold must be detached from a vulcanizing machine, and in particular, in the case that large rubber articles are molded, such an additional step is not negligible, so that productivity lowers inconveniently.
Furthermore, as the techniques of cleaning the mold without detaching it from the vulcanizing machine, there have been a method which comprises using, as a cleaning rubber, a rubber composition containing an amino-alcohol in Japanese Patent Application Laid-open No. 122942/1983, and a method which comprises applying an amino-alcohol or a cleaning solution obtained by dissolving the amino-alcohol in ethanol onto the soiled surface of the mold, and then vulcanizing an unvulcanized rubber composition in a usual manner to clean the mold (Japanese Patent Application Laid-open No. 180600/1983). In addition, many kinds of rubber compositions for cleaning the mold have been commercially available, and they are effective to some extent but they are not sufficient. Moreover, most of these cleaning rubber compositions contain a large amount of an amine agent which easily decomposes, and hence, even at room temperature, ammonia odor is given off. After heating, a large amount of an amine is further generated, and the generated amine corrodes the mold and gives rise to the problem of the pollution of a working environment.
Thus, the present inventors have suggested a rubber composition for cleaning a mold which can be obtained by blending an amino-alcohol, a glycol and an adsorbent with the intention of solving the above-mentioned problems, whereby the problems of the damage and the corrosion of the mold and the problem of the pollution of the working environment can be solved (Japanese Patent Application Laid-open No. 154850/1993). Furthermore, for the purpose of further increasing a cleaning effect by filling the cleaning rubber even into corners of the mold having an intricate shape, there has been suggested a rubber composition for cleaning the mold which can be obtained by blending an amino-alcohol, a glycol, an adsorbent and an organic peroxide (Japanese Patent Application Laid-open No. 128416/1994).
The amino-alcohol which is blended in these rubber compositions for cleaning the mold has the excellent mold cleaning effect.
However, these rubber compositions for cleaning the mold still have a problem that the amino-alcohol vaporizes even at room temperature at the time of kneading or when they are allowed to stand, thereby giving off the ammonia odor, with the result of polluting the working environment. Furthermore, while the compositions are allowed to stand, the amino-alcohol vaporizes with the lapse of time, so that these compositions can not be allowed to stand for a long period of time. Thus, they have a practical problem that their uses are limited and the mold cleaning effect cannot be maintained for a long time.